Rechargeable electrochemical cells that use a nickel-hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode are well known in the art. In fact over the past several years, metal hydride cells have gained widespread market acceptance due to the fact that they incorporate highly desirable performance characteristics. Examples of these desirable characteristics include high charge acceptance, relatively long-cycle life and operation over a wide range of temperatures. Each of these performance characteristics represent improvements over the nickel cadmium and other battery systems known in the prior art.
Typically, the metal hydride hydrogen storage electrode is the negative electrode in a hydrogen storage system. The negative electrode material (M) is charged by the electrochemical absorption of hydrogen, and the electrochemical evolution of a hydroxyl ion. The reaction which takes place at the metal hydride electrode may be described according to the following formula: ##STR1##
The reaction that takes place at the positive electrode of a nickel metal hydride cell is also a reversible reaction. In the case of a nickel hydroxide electrode, the positive electrode reaction is as follows: EQU Ni(OH).sub.2 +OH.sup.- .rarw..fwdarw.NiOOH+H.sub.2 O+e.sup.-
The negative electrode of most metal hydride electrochemical cells can be characterized by one of two chemical formulas: The first is AB.sub.2, which describes TiNi type battery systems such as described in, for example, U.S. Pat. No. 5,277,999. The second formula is AB.sub.5 which describes LaNi.sub.5 type systems as described in, for example, U.S. Pat. No. 4,487,817.
Substantially all metal hydride electrochemical cells fall into one of these two categories. However, with respect to both of these types of materials, it has been found that the failure mode is usually the result of degradation of the metal hydride electrode. This degradation has been ascribed to the growth of a surface oxide film on the surface of the metal hydride electrode. The oxide film reduces the active area of the electrode, thus reducing the available area for the hydrogen reduction/oxidation reaction to occur. Since the total current has to be distributed over a smaller total area, the current density on the active surface increases. As a consequence, the rate of formation of the irreversible oxide layer increases. The internal resistance of the electrode also increases, further hastening failure of the electrode.
Moreover, the power density of metal hydride cells is not as great as in some other types of cells, notably nickel cadmium. Accordingly, metal hydride cells have not been appropriate for several applications, such as power tools.
Prior attempts to address these problems have focused mainly on the addition of more and more modifier elements to the hydrogen storage alloy material which makes up the metal hydride electrode. For example, many current examples of metal hydride materials include ten or more components mixed in varying ratios. As with any alloy, adding new elements to the hydrogen storage material increases complexity of the formation process, and adds to the cost of the overall material.
Accordingly, there exists a need to provide a means by which to reduce the formation of surface oxides on the surface of the metal hydride electrode and in the metal hydride electrochemical cells. The means for reducing oxide formation should be relatively simple, and not necessitate the use of additional elements added to the hydrogen storage alloy. Further, a need exists for metal hydride cells having relatively high power densities and capacities.